The IMS (IP Multimedia Subsystem) is a telecommunication architecture designed in order to provide multimedia services between mobile/fixed network users, using IP (Internet Protocol) network technology. In particular, the IMS enables the evolution from the concept of traditional circuit switched voice call over PSTNs/PLMNs (Public Switched Terrestrial Networks/Public Landline Mobile Networks) to the concept of packet switched multimedia session over IP networks. With IMS, a user will be able to establish multimedia sessions with other users and/or with applications (text, audio, video, content sharing, gaming, etc.), according to interaction logics and models controlled by the IMS architecture. To this purpose, IMS consists of several network elements that implement and support many signaling transactions (based on SIP, or Session Initiation Protocol), in order to allow the handling of the multimedia sessions experienced by the network's users.
The 3GPP (3rd Generation Partnership Project) has specified in different documents, called Technical Specifications (i.e. TS), the IMS architecture, identifying several functions and defining specific network elements that cooperate in order to provide such functions. In particular, TS 23.228 defines the stage-2 service description for the IMS, which includes the elements necessary to support IP multimedia services, and TS 23.002 describes functional elements of a PLMN, among which the functional elements of an IMS architecture.
FIG. 1 shows a portion 100 of a known IMS architecture, as derivable from the above cited documents. The IMS architecture portion 100 of FIG. 1 comprises: a User Equipment (UE) 101; a Call Session Controlling Function (CSCF) 102, subdivided in a Proxy-CSCF (P-CSCF) 102a, an Interrogating-CSCF (I-CSCF) 102b, a Serving-CSCF (S-CSCF) 102c; a Home Subscriber Server (HSS) 103; an Application Server (AS) function 104, typically comprised of several application servers AS1, AS2, . . . , ASN; a Service Locator Function (SLF) 105.
A brief description of the functions disclosed in FIG. 1 is given in the following, as taken from TS 23.002.
HSS
The HSS is the master database for a given user. It is the entity containing the subscription-related information to support the network entities actually handling calls/sessions. A network may contain one or several HSSs: it depends on the number of mobile subscribers, on the capacity of the equipment and on the organization of the network. As an example, the HSS provides support to the call control servers in order to complete the routing/roaming procedures by solving authentication, authorization, naming/addressing resolution, location dependencies, etc.
The HSS is responsible for holding the following user related information:                User identification, numbering and addressing information;        User security information: network access control information for authentication and authorization;        User location information at inter-system level: the HSS supports the user registration, and stores inter-system location information, etc.;        User profile information.        
The HSS also generates user security information for mutual authentication, communication integrity check and ciphering. Based on this information, the HSS is also responsible to support the call control and session management entities of the different subsystems of the operator's network. The HSS may integrate heterogeneous information, and enable enhanced features in the core network to be offered to the application and services domain, at the same time hiding the heterogeneity.
The HSS typically comprises the following functionalities:                IP multimedia functionality, to provide support to control functions of the IMS such as the CSCF. It is needed to enable subscriber usage of the IMS services. This IP multimedia functionality is independent of the access network used to access the IMS.        The subset of the HLR/AUC (Home Location Register/Authentication Center) functionality required by the PS (Packet Switched) domain of the network.        The subset of the HLR/AUC functionality required by the CS (Circuit Switched) domain of the network, if it is desired to enable subscriber access to the CS domain or to support roaming to legacy GSM/UMTS (Global System for Mobile communications/Universal Mobile Terrestrial System) CS domain networks.CSCF        
The CSCF can act as Proxy CSCF (P-CSCF), Serving CSCF (S-CSCF) or Interrogating CSCF (I-CSCF). The P-CSCF is the first contact point for the UE within the IMS; the S-CSCF actually handles the session states in the network; the I-CSCF is mainly the contact point within an operator's network for all IMS connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator's service area.
AS
An Application Server (AS) offers value added IP multimedia services and resides either in the user's home network or in a third party location. The third party could be a network or simply a stand-alone AS. The AS can communicate with the HSS.
The S-CSCF to AS interface is used to provide services residing in an AS. Two cases were identified:                Serving-CSCF to an AS in the home network.        Serving-CSCF to an AS in a trusted external network (e.g., third party or visited).        
An Application Server may influence and impact the SIP session on behalf of the services supported by the operator's network. An AS may host and execute services.
SLF
The SLF:                is queried by the I-CSCF during the registration and session setup to get the name of the HSS containing the required subscriber specific data; Furthermore the SLF is also queried by the S-CSCF during the registration.        is queried by the AS to get the name of the HSS containing the required subscriber specific data;        is queried by the 3GPP AAA (Authentication, Authorization and Accounting) server to get the name of the HSS containing the required subscriber specific data.        
The SLF is not required in a single HSS environment. Moreover, use of SLF is not required when AS are configured/managed to use pre-defined HSS.
FIG. 1 also shows some of the interfaces, or reference points, defined in the above standard documents, for reducing interoperability issues in case of use of equipment provided by different vendors. A brief description of the reference points disclosed in FIG. 1 is given in the following, still taken from TS 23.002.
Reference Point CSCF—UE (Gm Reference Point)
The Gm reference point supports the communication between UE and IMS, e.g. related to registration and session control. The protocol used for the Gm reference point is SIP.
Reference Point UE—AS (Ut Reference Point)
The Ut interface resides between the UE and a SIP Application Server. The Ut interface enables a user to manage information related to his services, such as creation and assignment of public service identities, management of authorization policies that are used, e.g., by Presence service, conference policy management, etc.
Reference Point CSCF—AS (ISC Reference Point)
This interface between CSCF and the AS is used to provide services for the IMS.
Reference Point HSS—CSCF (Cx Reference Point)
The Cx reference point supports information transfer between CSCF and HSS. The main procedures that require information transfer between CSCF and HSS are:    1) Procedures related to Serving CSCF assignment.    2) Procedures related to routing information retrieval from HSS to CSCF.    3) Procedures related to authorization (e.g., checking of roaming agreement).    4) Procedures related to authentication: transfer of security parameters of the subscriber between HSS and CSCF.    5) Procedures related to filter control: transfer of filter parameters of the subscriber from HSS to CSCF.Reference Point CSCF—SLF (Dx Reference Point)
This interface between CSCF and SLF is used to retrieve the address of the HSS which holds the subscription for a given user. This interface is not required in a single HSS environment.
Reference Point HSS—AS (Sh Reference Point)
The Application Server may communicate with the HSS. The Sh interface is used for this purpose.
Reference Point AS—SLF (Dh Reference Point)
This interface between AS and SLF is used to retrieve the address of the HSS which holds the subscription for a given user. This interface is not required in a single HSS environment.
TS 23.228 comprises a section dedicated to a HSS resolution mechanism, which enables the I-CSCF, the S-CSCF and the AS to find the address of the HSS that holds the subscriber data for a given user identity when multiple and separately addressable HSSs are deployed by the network operator. When more than one independently addressable HSS is utilized by a network operator, the HSS where user information for a given subscriber is available has to be found. To get the HSS name the I-CSCF and the S-CSCF query the SLF entity. The SLF is accessed via the Dx interface or via the Dh interface.
For example, on REGISTER and on mobile terminated INVITEs, the I-CSCF queries the HSS for user's subscription specific data, e.g. the actual location or authentication parameters. This also has to be accomplished by the S-CSCF on REGISTER.